Some integrated circuit make use of voltages greater than its supply voltage. This is especially the case for a circuit having a memory that requires high voltages to store and/or erase data. Several high voltages may be used. Conventionally, two approaches are used. A first approach includes the use of a charge pump that provides a voltage regulated as a function of the desired voltage. A second approach includes the use of several charge pumps and a selector circuit to switch over the different voltages, such as, for example, those indicated in the European application EP-A-0 750 397.
The selector circuits of the prior art use the highest voltage to for biasing so as to prevent any malfunctioning. Indeed, the switching over of positive voltage is generally done by a biased P channel MOS transistor. To prevent a reverse conduction of the P channel transistors, it is necessary that the well should be biased by a voltage at least equal to the highest voltage that could cross the P channel transistor. This because there is a risk that the reverse conduction of an output transistor may cause a collapse of the output voltage.
Since integrated circuits are becoming increasingly complex, it is becoming necessary to test increasing numbers of functional elements independently. To be able to test an integrated circuit independently of its charge pump, it proves to be worthwhile to make use of an external voltage to replace the output voltage of the charge pump. A problem then arises of determining the supply voltage of the selector circuit. Indeed, the circuit has to work independently. It is necessary that the charge pump should supply the selector circuit. Furthermore, in the test mode, it is necessary to use the external voltage which may be greater than the charge pump voltage.